In low power consumption communication applications, such as Internet of Things (IoT) or Bluetooth Low Energy (BLE), an RC oscillator is usually used instead of a crystal oscillator as a clock source in a sleep state of a system. To ensure accuracy of sleep timing of the system and reduce extra power consumption introduced by the system guard time, such applications generally have higher requirements on frequency accuracy of the RC oscillator. For example, the BLE requires that a sleep timer has a frequency deviation within 500 ppm.
Here, basic working principle of an RC relaxation oscillator is introduced. A capacitor is charged with a fixed current, and a voltage across the capacitor is compared with a reference voltage by a comparator. When the voltage across the capacitor is greater than the reference voltage, the comparator output is inverted, and the capacitor is discharged. Afterward, the charging is performed. The above process is repeated, so as to obtain a clock output signal with a specific frequency. The frequency of the clock output signal is related to (usually inversely proportional to) resistance and capacitance in the oscillator.
Referring to FIG. 1, FIG. 1 is a relaxation oscillation circuit in existing techniques. The circuit compensates for a mismatch voltage of a comparator by periodically switching positive and negative input terminals of the comparator. When an output clock is low, switches S1, S4 and S6 are turned on, a voltage across a capacitor C2 is 0, a current source charges a capacitor C1, V1 is a voltage across the capacitor C1, and V2 is a reference voltage. When V1 is charged to be greater than V2, an output of the comparator is inverted, and the output clock becomes high. At the time, the switches S2, S3 and S5 are turned on, and the capacitor C1 is discharged to 0 V. The current source charges the capacitor C2, V2 is a voltage across the capacitor C2, and V1 is a reference voltage. When V2 is charged to be greater than V1, the output clock is inverted to be a low level. By analogy, a periodic clock output is generated.
The above solution reduces correlation between frequency and temperature by using a resistor with a lower temperature coefficient. There are three specific ways: 1) using the combination of a resistor with a positive temperature coefficient and a resistor with a negative temperature coefficient to form the resistor with the lower temperature coefficient; 2) using a resistor with an ultra-low temperature coefficient in special processes; 3) using an off-chip resistor with a low temperature coefficient.